A cellular telephone system is a radio communication system in which a plurality of mobile stations operate within a cell which is serviced by a base station. There are typically a relatively large number of cells and base stations in a given service area. The base station in each cell is connected to a message switching center which in turn is connected to a public telephone system.
The first cellular mobile systems placed in public use were analog systems typically used for speech or other types of analog information. These systems include multiple radio channels for transmitting analog information between base and mobile stations by transmitting analog modulated radio signals. The analog systems are being replaced by dual mode systems which are capable of operating in either a digital or analog mode of operation. The Electronic Industry Association (EIA) has published EIA/TIA Project No. 2398, entitled "Cellular System, Dual-Mode Mobile Station-Base Station Compatibility Standard", IS-54 (Revision A), dated January 1991; and TR45.3, Project No. 2216, entitled "Cellular System, Recommended Minimum Performance Standards for 800 MHz Dual Mode Mobile Stations," dated March 1991. These Interim Standards provide specifications for the design of a cellular telephone system which is capable of operating in both analog and digital modes.
The emerging mobile telephone system, which is commonly referred to as the ADC-System, will work in both analog and digital modes. A telephone operating in the analog mode will have a power stage that will work continuously at a power level determined by the base station, and the modulation method will be FM. The maximum allowed output power of a mobile station will depend on the class of the mobile station. Presently, there are four classes I, II, III and IV, and the maximum output power of the classes are +6, +2, -2, and -2 dBW ERP, respectively. Accordingly, such a system advantageously employs a non-linear transmitter power stage having a high power efficiency. The EIA has specified that the digital mode will have a modulation method which will include a substantial amplitude variation in the transmitted signal (.pi./4-DQPSK). The amplitude variation has an order of magnitude of approximately 20 dB, and the quotient between the peak and average power is about 4 dB. Accordingly, the power stage for the digital mode will have to be linear, and it will have to be dimensioned for a higher peak power. The problem with a completely linear power stage is that the power efficiency is relatively low. The specified digital mode, however, only requires that the transmission take place in one of three time slots in a full rate mode or in one of six time slots in a half rate mode. Because the transmission occurs in different time slots, the current consumption is not of utmost importance.
It is desirable that the transmitter power amplifier of a telephone in the ADC-system be able to operate (1) continuously in the analog mode with high power efficiency, and (2) in the digital mode where there are restrictive requirements on power level and linearity. Accordingly, there is a need for a transmitter power amplifier that can efficiently meet the requirements of analog and digital operation in an ADC-system.